As additional information regarding cell-to-cell recognitions among oral bacteria is discovered, it is becoming increasing clear that a dynamic but organized microbial community exists in the oral cavity. The part of this complex community studied in this laboratory pertains especially to microbial ecology. Many of the bacteria that commonly inhabit this region were tested by an in vitro simulated plaque model. In the presence of potential competitors, several strains were tested as examples to support certain principles of coaggregation. Results of this study indicated that the oral cavity is replete with competitors and cohorts. Strains that belonged to different genera could recognize common third partners and prevented each other from coaggregating with them. Alternatively, demonstrations of multigeneric bacterial complexes were commonplace, indicating the similarity to associations of bacteria found in dental plaque. The surface structures that mediate these coaggregations were studied by using bacteriophage as surface probes and by isolating coaggregation-defective mutants. Identity of one receptor with a mediator of coaggregation was shown by altered coaggregation properties of bacteriophage-resistant mutants compared to their parent strains. To aid in further study of these surface structures, eight new actinomyces phage were isolated for the first time from human dental plaque. Studies with coaggregation-defective mutants revealed that certain outer membrane or surface proteins were altered or missing in the mutants but present in the parent strain. Results of a survey of the coaggregation properties of nearly 100 strains of Fusobacterium, Selenomonas, and Veillonella indicated that fusobacteria were very reactive, selenomonas hardly recognized any partners, and veillonella exhibited many lactose-inhibitable coaggregations. The results of each of these investigative approaches are focused on understanding the relationship of cell surface recognitions among oral bacteria and their role in microbial ecology.